1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer, and more particularly to a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer consisting of at least two layers.
2. Description of Prior Art
A magnetic recording medium has been widely used as a tape for recording music (i.e., an audio tape), a video tape and a floppy disc. The magnetic recording medium basically comprises a nonmagnetic support and a magnetic recording layer provided on the support, and the magnetic recording layer comprises a binder and a ferromagnetic powder dispersed therein.
The magnetic recording medium is desired to show high levels in various properties such as electromagnetic conversion characteristics, running endurance and running property. Particularly, the recording medium is desired to show excellent electromagnetic conversion characteristics. For example, an audio tape is required to have high reproducibility of original sounds, and a video tape is required to have high reproducibility of original images. Such high electromagnetic conversion characteristics is desired particularly in the case of an 8 mm type-video tape recorder.
It is known that the electromagnetic conversion characteristics of the magnetic recording medium using a ferromagnetic powder greatly varies depending upon dispersibility (or dispersed condition) of the ferromagnetic powder in the magnetic recording layer. In more detail, even if a ferromagnetic powder having excellent magnetic property is used for the purpose of enhancing the electromagnetic conversion characteristics of the resulting medium, the excellent magnetic property of the ferromagnetic powder is not reflected on the enhancement of the electromagnetic conversion characteristics when the ferromagnetic powder is poorly dispersed in the recording layer.
For improving the dispersibility of the ferromagnetic powder in the magnetic recording layer, there has been utilized a method of kneading or dispersing the ferromagnetic powder for a long period of time in the preparation of a magnetic paint for forming a magnetic recording layer. However, such long-time kneading or dispersing causes deterioration of the magnetic property of the ferromagnetic powder, and hence a measure of incorporating a polar group into a resin component used in a binder of a recording layer is recently utilized so as to make the binder show a favorable affinity for the ferromagnetic powder.
For example, Japanese Patent Provisional Publication No. 59(1984)-5424 discloses a magnetic recording medium in which a resin component having a specific polar group such as a metal sulfonate group is employed in an amount of not less than 50 wt.% as a binder of the recording layer using a ferromagnetic metal powder. By employing such resin having a specific polar group as a resin component of the binder, the ferromagnetic metal powder can be well dispersed in the magnetic recording layer, whereby the resulting recording medium can be highly improved in the electromagnetic conversion characteristics.
With respect to the molecular weight of a resin used in a binder, even for the above-mentioned resins having a specific polar group, it is known that a resin of low-molecular weight can better disperse the ferromagnetic metal powder therein and thereby improve the electromagnetic conversion characteristics of the resulting medium, as compared with other resins of high-molecular weight.
It is possible to prepare a magnetic recording medium improved in the electromagnetic conversion characteristics to a certain level as described above. However, further improvement of magnetic recording medium in both of the electromagnetic conversion characteristics and the running endurance is desired so far.